This document contains parts, sometimes verbatim, from the original antenna tutorial included in CST help documentation

CST Microwave Studio Patch Antenna Tutorial

In this tutorial you will learn how to simulate basic antenna devices. We will design and simulated one of the most widely used antenna architectures, the rectangular patch antenna. The following explanations can be applied to other antennas as well.

Although, CST MICROWAVE STUDIO® can provide a wide variety of results, this tutorial will concentrate mainly on the S-parameters and farfield results.

Index

1. Project and Template 2. Geometric model 3. Excitation, frequency range, boundaries and monitors. 4. Solver 5. Results

1. Project and Template

After you have started CST DESIGN ENVIRONMENT™ and have chosen to create a new CST MICROWAVE STUDIO® project, you are requested to select a template that best fits your current device. Here, the Antenna (on Planar Substrate) template should be chosen. If the following dialog box does not occur automatically, select File Select template... from the main menu.

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This template automatically sets the units to mm and GHz, defines the background material to vacuum and selects appropriate boundary conditions (see chapter Boundary Conditions). Because the background material has been set to vacuum, the structure can be modeled just as it appears on your desk.

2. Geometric Model

Now, we start creating the 3D model of the patch antenna. One of the most powerful characteristics of CST it that the 3D models are parametric. This is particularly useful when optimizing the antenna since it is enough to change the value of these parameters to change the dimensions of the antenna model.

Lets define our first three parameters. These parameters will determine the length, width and heigth of the antenna substrate. They can be directly introduced in the Parameter List located on the lower part of the screen. If this list is not visible, it can be displayed at View Parameter list. Remeber that the default length units for this template are 'mm'. You can see the unit default values in the lower-left corner of the screen.

Now we can create the antenna substrate using the brick creation function (Objects Basic

Shapes Brick, ). Double-click in the design area to define the corners of the brick. Immediately a new window will appear where you can specify the position and dimensions of the brick.

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We specify the name of the brick: 'Substrate'. The coordinates of the brick can be specified

using either numerical dimensions or parameters. In this case, we will use the previously defined parameters. The material can be directly selected from a Material Library. We will use a standard substrate Rogers RO4003(lossy).

We can follow the same process to include the ground plane and the patch antenna. For

the patch antenna, we will need to additional parameters to define its length and width:

Notice that 32mm corresponds to half a wavelength at 2.5 GHz considering the RO4003

permitivity. We'll choose equal Zmin and Zmax and CST will model the brick as a 2D surface. Due to the

small height of copper metallization (35um typically) a 2D modelling is accurate and faster than simulating a 3D model. The material used in this case will be copper.

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The basic structure of the patch antenna is already created. You can now modify the value

of any parameter (PatchL, for instance), click F7 and the model will be automatically updated. To excite the patch antenna we need also a feeding structure. We will use a coaxial

excitation. The coaxial structure is composed of an inner conductor, a dielectric and an outer conductor. Each one of the elements is created using the cylinder creation mode (Objects

Basic Shapes Cylinder, ). We will need the following parameters to specify the radius of the cylinder, their length and position. The correspond to a commercial SMA connetor.

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For the dielectric we will use a custom defined Teflon as dielectric:

After creating the coaxial feeding the antenna model should look like the following:

To finish the 3D model, the last thing to do consists on removing the part of the ground plane that is shortcircuiting the coaxial feed. Select the ground plane and click Objects

Boolan Insert ). Then select the 3 elements of the coaxial feed (inner conductor, dielectric and outer conductor) and press enter. Now the ground plane has a circular hole to prevent blocking the RF signal travelling from the coaxial and the patch antenna model is complete.

Some solvers require that back of the RF-ports is covered by a block of Perfect Electric

Conductor PEC. This can be easily done with the brick tool.

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3. Excitation, frequency range, boundaries and monitors.

To this point, only the structure itself has been modeled. Now it is necessary to define some solver-specific elements. For an S-parameter calculation you must define input and output ports. Furthermore, the simulation needs to know how the calculation domain should be

terminated at its bounds.

Excitation

The next step is to add the excitation port to the patch antenna device, for which the reflection parameter will later be calculated. The port simulates an infinitely long coaxial waveguide structure that is connected to the structure at the ports plane.

The easiest way to define the port is to pick the face (Objects Pick Pick Face, ) of the coaxial feed (the dielectric element) and open the waveguide dialog box (Solve Waveguide

Ports, ) to define the port:

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Here, you have to choose how many modes should be considered by the port. For a simple coax port with only one inner conductor, usually only the fundamental TEM mode is of interest. Thus, you should simply keep the default setting of one mode. We can also specify the position of the reference plane. In this case, we want the reference plane to be located at the antenna ground plane and therefore the distance to the ref. plane is –CoaxL. The position of this ground plane basically affects the phase of the reflection coefficient. Frequency range In this example, the S-parameters are to be calculated for a frequency range between 2 and 3

GHz. Open the frequency range dialog box (Solve Frequency, ) and enter the range from 2 to 3 (GHz) before pressing the OK button (the frequency unit has previously been set to GHz by the selected template and is displayed in the status bar). Boundaries Because the calculation domain is only a limited volume it is necessary to define boundary conditions that incorporate the influences of the outer space. Please open the Boundary

Conditions dialog box by selecting Solve Boundary Conditions ( ).

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In this window, different boundary conditions such as electric, magnetic or periodic boundaries can be defined, as well as symmetry planes that can speed up the simulation. In this case, the default options are used (open boundaries and no symmetry planes). Monitors

Besides the S-parameters, the main result of interest for antenna devices is the farfield distribution at a given frequency. The solvers in CST MICROWAVE STUDIO® offer the possibility of defining several field monitors to specify at which frequencies the field data shall be stored.

Please open the monitor definition dialog box by selecting Solve Field Monitors ( ):

In this dialog box, you should first select the Type Farfield/RCS before specifying the frequency for this monitor in the Frequency field. Afterwards, press the Apply button to store the monitors data. Please define a monitor at the frequency of 2.4 (with GHz being the currently active frequency unit). However, you may define additional monitors at other frequencies, each time pressing the Apply button to confirm the setting and add the monitor in the Monitors folder in the navigation tree. After the monitor definition is complete, please close this dialog box by pressing the OK button.

4. Solver

CST MICROWAVE STUDIO® has several solvers. The performance of each solver depends on the type of antenna problem (electrical size, presence of dielectrics, bandwidth...) . For this patch antenna we will use the transient solver. This is a very flexible time domain simulation module that is capable of solving any kind of S-parameter or antenna problem. It will stimulate the structure at a previously defined port using a broadband signal. Broadband

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stimulation enables you to receive the S-parameters for your entire desired frequency range and, optionally, the electromagnetic field patterns at various desired frequencies from only one calculation run.

Open the solver parameters dialog box by selecting Solve Transient solver ( ).

We recommend using reasonably large bandwidths of 20% to 100% for the transient simulation. In this example, the S-parameters are to be calculated for a frequency range between 2 and 3 GHz. With a center frequency of 2.5 GHz, the bandwidth (3 GHz - 2 GHz = 1 GHz) is 40% of the center frequency, which is inside the recommended interval. Thus, you can simply choose the frequency range as desired between 2 and 3 GHz.

The accuracy of the method is basically determined by two factors:

(1) The accuracy in the processing of the time signals. It is basically determined by the duration of the time simulation and is specified by the Accuracy [dB].

(2) The quality of the mesh. The finer the mesh, the more accurate will be the resuly, but the longer the simulation time will be. For an accurate simulation, it is convenient to turn on the adaptive mesh refinement option. This way the mesh will be automatically refined until converge is reached.

In this example we use the default values which provide a good accuracy for this simple structure:

Accuracy = -30 dB Adaptive mesh = OFF

The simulation of this structure using the previous solver parameters should take 1 or 2 minutes.

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During the simulation, the message window will display additional information:

5. Results

You have simulated the rectangular patch antenna using the transient solver. Lets review the results.

Port Signals

First, observe the port signals. Open the 1D Results folder in the navigation tree and click on the Port signals folder. It is possible to observe the progress of the results during the computation. In order to get the complete information, however, wait until the solver has finished.

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This plot shows the incident and reflected wave amplitudes at the waveguide port versus time. The incident wave amplitude is called i1 (referring to the port name: 1) and the reflected wave amplitude is o1,1. As evident from the above time-signal plot, the patch antenna has a strong resonance that leads to a slowly decreasing output signal.

By comparing the Fourier the transform of these two signals, the S-parameters of the antenna can be computed.

S-parameters

A primary result for the antenna is the S11 parameter that will appear if you click on the 1D Results |S| dB folder from the navigation tree. The following screenshot shows the reflection parameter:

It is possible to precisely determine the operational frequency for the patch antenna. Activate the axis marker by pressing the right mouse button and selecting the Show Axis Marker option from the context menu. Now you can move the marker to the S11 minimum and pinpoint a resonance frequency for the patch antenna of about 2.45 GHz.

Port Field

It always important to check the E-fields on the port. You can easily display this field by opening the 2D/3D Results Port Modes Port1 folder from the navigation tree. To visualize the electric field of the port mode, please click on the e1 folder.

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In this case, as expected, the field corresponds to the dominant mode in a coaxial waveguide. The plot also shows some important properties of the mode such as mode type, propagation constant and line impedance. Here, you can see that the port impedance is approximately 50 Ohms, which is agrees with the specifications of the SMA connector.

Radiation pattern

The farfield is another important parameter in antenna design. The farfield solution of the antenna device can be shown by selecting the corresponding monitor entry in the Farfields folder from the navigation tree. For example, the farfield at the frequency 2.4 GHz can be visualized by clicking on the Farfields farfield (f=2.4) [1] entry, showing the directivity over the phi and theta angle (the position of the color ramp can be modified by clicking on the Results Plot Properties Color ramp button):

You can see that the maximum power is radiated in the positive z-direction. Note that there are several other options available to plot a farfield: the Polar plot, the Cartesian plot and the 2D plot.

You can also obtain the maximum directivity, which is 7.5dB in this case, as well as the radiation efficiency of the structure, -0.85dB (82%).

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Congratulations! You have just completed the antenna tutorial that should have provided you with a good working knowledge on how to use CST MICROWAVE STUDIO® to calculate S-parameters and farfield results.

In addition to this tutorial, you can find more S-parameter calculation examples for planar structures or various antenna models in the Examples folder in your installation directory. Each of these examples contains a Readme item in the navigation tree that provides more information about the particular device.

Finally, you should refer to the Online documentation for more in-depth information on issues such as the fundamental principles of the simulation method, mesh generation, usage of macros to automate common tasks, etc.